1. Field of the Invention
The present invention relates to a method of regenerating a phosphor from a recovered phosphor slurry containing carbon, a bichromate, and polyvinyl alcohol and, more particularly, to a method of reclaiming a phosphor for a cathode-ray tube.
2. Description of the Related Art
In general, red, blue, and green emitting phosphors are aligned as dots or stripes on the inner surface of a faceplate to form a phosphor screen of a color cathode-ray tube. Examples of the color cathode-ray tube phosphor are a copper-activated zinc sulfide phosphor, a copper and gold-coactivated zinc sulfide phosphor as a green emitting phosphor, a silver-activated zinc sulfide phosphor as a blue emitting phosphor, and a europium-activated yttrium oxide sulfide phosphor and a europium-activated yttrium oxide phosphor as a red emitting phosphor.
A typical example of a method of forming a phosphor screen is a slurry method. In this slurry method, a phosphor is dispersed in a photosensitive aqueous solution mixture consisting of polyvinyl alcohol (PVA), ammonium bichromate (ADC), and a surfactant to form a phosphor slurry, and the slurry is uniformly coated on the inner surface of a faceplate of a color cathode-ray tube by using, e.g., a spin coater. The coated slurry is exposed through a shadow mask to form a predetermined pattern, a portion of the phosphor in the exposed portion of the slurry is fixed on the faceplate, and the remaining phosphor slurry is washed away. A series of these steps are repeatedly performed for each of blue, green, and red emitting phosphors, thereby forming a phosphor screen. In a so-called black matrix type cathode-ray tube, dots or stripes of a black substance such as carbon or black chromium are formed on the inner surface of a faceplate beforehand, and then the above steps are repeatedly performed to form a phosphor screen.
Since an amount of a phosphor washed away in the above slurry method is about 70% or more of a used phosphor amount, a washed away portion of the phosphor should be normally recovered and reused. In particular, it is important to reuse a phosphor containing an expensive rare earth element as in a yttrium-based red emitting phosphor.
A recovered phosphor slurry, however, contains not only phosphor slurry components such as ADC and PVA but also extraneous materials such as a lubricating oil of, e.g., a spin coater or a recovering machine, a black substance such as carbon or black chromium and a phosphor of another color emitting peeled from a formed phosphor screen, foreign substances, and dust. For example, a red emitting phosphor slurry which is recovered after the last coating step of the slurry method contains, in addition to slurry components such as a red emitting phosphor, PVA and ADC, a small amount of extraneous materials such as carbon, green and blue emitting phosphors, and an oil component. When such a recovered slurry is directly reused, PVA is solidified, an oil component having volatility volatilizes upon formation of a phosphor screen, or an oil component not having volatility causes nonuniform coating. Therefore, it is difficult to form an uniform phosphor screen. In addition, since other color emitting phosphors and a black substance are contained, a color tone and image quality are degraded. For this reason, as a recovered phosphor reclaiming method capable of removing these undesired materials, various types of methods have been conventionally proposed.
For example, Japanese Patent Application No. 47-557 discloses a method in which an alkali solution having a pH of 12 or more and a hypohalogenous acid salt such as sodium hypochlorite (NaOCl) are added to a recovered phosphor slurry, a rare earth phosphor is separated from a suspension obtained by heating the resultant phosphor slurry and is washed with water, thereby reclaiming a phosphor. According to this method, however, since a large amount of NaOCl is used at a high alkali concentration, the surface of a phosphor is significantly oxidized, and the brightness of a reclaimed phosphor is significantly decreased.
Published Unexamined Japanese Patent Application No. 53-17587 or 53-30486 discloses a method in which an alkali is added to a recovered phosphor slurry, and the slurry is washed with water and a phosphor is separated to be recovered after the slurry is heated up to 60.degree. C. Since, however, silica or the like which is coated on the surface of a phosphor is dissolved or peeled together with, e.g., PVA by an alkali treatment, dispersibility of a phosphor in a phosphor slurry and its adhesive strength with respect to a faceplate are reduced.
In addition, Published Unexamined Japanese Patent Application No. 53-18489 discloses a method in which an alkali-treated phosphor is baked at 200.degree. C. to 400.degree. C., dissolved or peeled silica is adhered on the phosphor again, and the phosphor is washed with water and separated to be recovered. Although a reduction in adhesive strength of the phosphor reclaimed by this method is improved, however, its dispersibility is still unsatisfactory.